Magnetic and structural investigation of growth induced magnetic anisotropies in Fe50Co50 thin films

Thin Fe50Co50 (FeCo) films are highly magnetostrictive and have recently attracted great attention due to their possible application in magnetic devices. The growth conditions may highly affect the FeCo films magnetization reversal process as they may induce compressive or tensile stress and additional magnetic anisotropies, accordingly [1]. We present FeCo layers, with thickness, t, ranging from 5 up to 100 nm, grown by dc-magnetron sputtering on Si substrates in Ar atmosphere. RT magnetization data were collected using a SQUID magnetometer and a MOKE apparatus, using both an in-plane and an out-of-plane magnetic field. Magnetoresistance (MR) measurements were collected using the Van der Pauw method. The structural characterization was performed with a high resolution X-Ray diffractometer and an optical profilometer. For t ≤ 20 nm, the shape of the in-plane M loops is squared and coercivity increases with t, possibly due to the grain size growth induced by the thickness increase. The MR response is anisotropic in character. For t > 20 nm, coercivity smoothly decreases and the shape of the loops changes, the approach to saturation gets slower and the shape of the whole loop gets less and less squared. The MR effect becomes almost isotropic and its intensity increases of about one order of magnitude. These results suggest that the magnetization reorientation process changes for t > 20 nm, and are in agreement with the progressive development of an out-of-plane easy axis [2]. This hypothesis is substantiated by profilometric analysis that reveals an increase in samples curvature with t, thus confirming a change in the degree of in-plane tensile stress. Diffraction data collected for different t values will be presented and discussed, as well. [1] W. Yu et al., J. Appl. Phys. 99 (2006) 08B706. [2] G. Ausanio et al., Thin Solid Films 519 (2011) 5420.